Destruction of C2H4O2 isomers in ice-phase by X-rays: Implication on the abundance of acetic acid and methyl formate in the interstellar medium. (1st December 2017)

• Record Type: Journal Article
• Authors: Rachid, Marina G.; Faquine, Karla; Pilling, S.
• Is Part Of: Planetary and space science. Volume 149(2017)
• Page Start: 83
• Page End: 93
• Publication Date: 2017-12-01
• Subjects: Astrochemistry -- Methyl formate -- Acetic acid -- Laboratory measurements -- Ices -- Soft X-rays; Space sciences -- Periodicals; Atmosphere, Upper -- Periodicals; Sciences spatiales -- Périodiques; Haute atmosphère -- Périodiques; 523
• DOI: 10.1016/j.pss.2017.05.003
• Languages: English
• ISSNs: 0032-0633

Abstract

Abstract: The C2 H4 O2 isomers methyl formate (HCOOCH3 ), acetic acid (CH3 COOH) and glycoaldehyde (HOCH2 CHO) have been detected in molecular clouds in the interstellar medium, as well as, hot cores, hot corinos and around protostellar objects. However, their abundances are very different, being methyl formate more abundant than the other two isomers. This fact may be related to the different destruction by ionizing radiation of these molecules. The goal of this work is experimentally study the photodissociation processes of methyl formate and acetic acid ices when exposed to broadband soft X-ray from 6 up to 2000 eV. The experiments were performed coupled to the SGM beamline in the Brazilian Synchrotron Light Source (LNLS/CNPEM) at Campinas, Brazil. The simulated astrophysical ices (12 K) were monitored throughout the experiment using infrared vibrational spectroscopy (FTIR). The analysis of processed ices allowed the determination of the effective destruction cross sections of the parent molecules as well as the effective formation cross section of daughter molecular species such as CO, CO2, H2 O, CH4 and H2 CO (only for methyl formate) and the hydrocarbons C2 H6 and C5 H10 (only for acetic acid). The half-lives of molecules at ices toward young stellar objects (YSOs) and inside molecular clouds (e.g. Sgr B2 and W51) due to the presence of incoming soft X-rays were estimated. We determined the effective formation rate and the branching ratios for assigned daughter species after the establishment of a chemical equilibrium. The main product from photodissociation of both methyl formate and acetic acid is CO, that can be formed by recombination of ions, formed during the photodissociation, in the ice surface. The relative abundance between methyl formate and acetic acid (NCH3COOH /NHCOOCH3 ) in different astronomical scenarios and their column density evolution in the presence of X-rays were calculated. Our results suggest that such radiation field can be one of the factors that explain the difference in the C2 H4 O2 isomers abundances. Highlights: We studied the photodissociation processes of methyl formate and acetic acid ices exposed to broadband soft X-ray (6–2000eV). The simulated astrophysical ices (12 K) were monitored throughout the experiment using FTIR spectroscopy. We calculated the effective destruction/formation cross section for parent/daughter species. The difference in the isomers abundances can be due to photodissociation process triggered by X-ray in highly embedded regions.